The first stanza deals with the incident itself. The first two lines with their rather unemotional, conversational tone, could be interpreted ironically, as the narrator just pretending to be sorry, but Armitage himself explained that he wants the reader “to see the tone of these opening lines as a lie which covers up real feelings”.
The second stanza deals with the results of the boy's actions and begins with
“O the unrivalled stench of branded skin
as you slipped your thumb and middle finger in”
By his actions, the boy has “marked” her, (note the fore grounded position of “marked” beginning a sentence, at the end of a line), “branded” her, linked her to himself forever, with a doctor, not a priest in attendance. It seems that at thirteen years old, he was unable to express tender feelings, and so was prepared to make her notice him in any way he could.
The final stanza provides some sort of explanation. It suggests that his act was the clumsy attempt of a boy of thirteen to convey his love, to claim her as his own. However, the negative phrasing of “Don't believe me” requires the reader to consider the meaning carefully.